1. Field of the Invention
The inventions discussed herein are related to an illumination optical system for an image pickup apparatus.
2. Description of the Related Art
Image pickup apparatuses designed to read information from a subject, such as palm-vein authentication apparatuses and barcode readers (hereinafter simply referred to as “image pickup apparatuses), are known kinds of image pickup apparatuses. An image pickup apparatus includes, for example, an illumination optical system 101 and an image pickup optical system 103, as illustrated in FIG. 1. A light source 102, such as an LED, irradiates a subject 106 with irradiation light, and an image sensor 105 provided on the image side of an image pickup lens 104 detects regular reflection light and scattered light which are both reflected by the subject 106 (e.g., a palm or a sheet on which a barcode is printed). Thereby, an image pickup apparatus 100 illustrated in FIG. 1 reads information of the subject 106.
In the meantime, as illustrated in FIG. 1, both regular reflection light (solid lines in FIG. 1) and scattered light (dashed lines in FIG. 1) having a lower light intensity than the regular reflection light are generated from each point on the surface of the subject 106 irradiated with irradiation light, but not all of the regular reflection light rays generated from the points in the field of view of the image pickup optical system 103 enter the image sensor 105. Thus, even when a point from which regular reflection light to be detected by the image sensor 105 is emitted (point B in FIG. 1) and a point from which regular reflection light not to be detected by the image sensor 105 is emitted (points A and C in FIG. 1) are illuminated at the same illuminance, the amount of detected light becomes different between these two points.
Such a difference in the amount of light caused by regular reflection light (hereinafter referred to as “regular-reflection noise”) is one factor that deteriorates the reading precision of the image pickup apparatus. Thus, in the field of image pickup apparatuses, a technology has been pursued for limiting regular-reflection noise so as to improve the reading precision of the image pickup apparatuses.
In order to improve the reading precision of the image pickup apparatus by limiting regular-reflection noise, it is necessary to prevent high-luminance regions from being locally generated on a subject, but simply equalizing illuminance is insufficient. The directivity of illumination light needs to be controlled to appropriately guide light generated from a subject to the image-pickup optical system. As a technology for controlling the directivity of light, a technology is known wherein the direction in which light is emitted is controlled using a prism plate, and such a technology is disclosed by, for example, patent document 1.
FIG. 2, FIG. 3, FIG. 4, and FIG. 5 illustrate controlling the direction in which light is emitted by a prism plate in accordance with a prior art. FIG. 2 illustrates a relationship between incident light and outgoing light on an XZ cross-section of a prism plate. FIG. 3 illustrates a relationship on an XZ cross-section of the prism plate between outgoing light and the light perpendicularly incident onto the prism plate. FIG. 4 illustrates a relationship on a YZ cross-section taken on line AA′ in FIG. 3 between outgoing light and the light perpendicularly incident onto the prism plate. FIG. 5 illustrates a relationship between incident light and outgoing light with the prism plate illustrated in FIG. 2 being viewed from above. The XYZ coordinate systems of FIG. 2 to FIG. 5 are right-handed orthogonal coordinate systems provided for the convenience of reference to directions.
A prism plate 200 illustrated in FIG. 2 includes: a prism surface 201 as an incidence surface on which a plurality of parallel prism columns each having a ridge line 202 are arranged in the X direction; and a plane surface 203 as an outgoing surface. The prism plate 200 allows incident light that enters from the prism-surface-201 side to be deflected by refraction at the prism surface 201 and the plane surface 203 so as to control the outgoing direction, with the result that the directivity of the outgoing light can be controlled.    Patent document 1: Japanese Laid-open Patent Publication No. 2004-171192